Teletext decoders of this kind include a data acquisition and control circuit which operates on serial data derived from the received video signal and converts it into data bytes which are stored. One of the functions of the data acquisition and control circuit is to detect that the serial data which is being received is valid teletext data, and to reject it if it is not teletext data.
Teletext transmission formats are described in the documents "Broadcast Teletext Specification", September 1976, published in U.K. jointly by the British Broadcasting Corporation, the Independent Broadcasting Authority, the British Radio Equipment Manufacturers' Association, and "World System Teletext and Data Broadcasting System", December 1987, complied by the Department of Trade and Industry of the U.K. Government. Teletext data is normally transmitted in those lines of a television signal which are included in the field or vertical blanking interval thereof, i.e. lines 7 to 22 and lines 318 to 335 of a 625 line, 50 Hz field rate system. However, some of these lines, such as lines 19 and 20 and lines 332 and 333 may be used to transmit Insertion Test Signals (ITS) and it has been found that these signals can mimic teletext data with the result that non-teletext "junk" data is received which corrupts the valid teletext data. Non-teletext "junk" data can also arise from the normal picture information contained on the received video signal.
In order to alleviate this problem, it is arranged that the data acquisition and control circuit carries out a number of tests on the received serial data in order to determine whether it is valid teletext data or not. One of these tests is to determine whether the so-called framing code which follows the clock run-in bits of each data packet of teletext data occurs within a predetermined "window". If a valid framing code does not occur within this "window" then the relevant serial data is not acquired. Normally the framing code "window" is generated using the line sync signal derived from the received video signal as a reference, it being known that a valid framing code should occur a predetermined time period following the line sync signal.
Even using such a test it is still possible for non-teletext "junk" data to appear to contain a valid framing code in the framing code window, and therefore be passed as valid teletext data. The present invention is concerned with reducing the possibility of this happening.
One particular form of data acquisition and control integrated circuit marketed by Philips Components -type SAA 5250, commonly referred to as CIDAC (CMOS Interface for Data Acquisition and Control) has been designed primarily for use in connection with the French ANTIOPE teletext system. Details of this integrated circuit can be found in Philips Data Handbook, "Video and associated systems --Bipolar, MOS", Book ICO2a, 1988 and also in "CIDAC-CMOS Interface for Data Acquisition and Control" by RTC Laboratories Applications, Paris. CIDAC may be used in two alternative modes. In one mode, referred to as SYNC, a framing code window is generated internally from a line sync signal which is applied to the VALI/SYNC input of CIDAC. In its alternative mode, the signal applied to the VALI/SYNC input of CIDAC affords the framing code window directly.